1. Field of the Invention
The present invention relates to a hydrophone array, particularly hydrophone arrays for underwater acoustic sensing of subsurface marine vehicles.
2. Background of the Art
Undersea noise is well known to be highly dynamic and anisotropic. Surface-generated noise undergoes multipath propagation in the ocean waveguide arriving at the receiver at a distribution of elevation angles. Discrete noise contributions from anthropogenic and biologic sources add acute directionally in azimuth and elevation. Shallow water bathymetry also contributes by varying the distribution of the noise field across azimuth. Such anisotropy in the noise field may be exploited to increase the detection range of a passive sonar array. However, detection performance can be severely affected by nearby shipping noise through the array's beam response sidelobes.
The detection performance of low-frequency passive sonars, may be severely affected if deployed under heavy shipping conditions. Merchant ships, tankers and other anthropogenic undersea acoustic sources are very loud and they may affect detection performance even at long distances from the receiver. However, energy from the distant ships (which drops by about 6 dB/octave in source level with increasing frequency) undergoes water-column absorption and multiple bottom interactions while the energy from a nearby quiet signal will be less attenuated due to its shorter range. Therefore, at higher frequencies, clutter from distant shipping noise is expected to decrease more rapidly than the received energy level of the target. In addition, high frequency arrays are much smaller, which reduces cost and makes them attractive components of unmanned vehicles and expendable systems.
There is yet need for array designs and signal processing approaches to satisfy a number of requirements: a high-frequency passive sonar to mitigate clutter from shipping noise. The sonar system should be easy to deploy and the number of array elements should be minimized to reduce data rate and processor demand. To maximize the detection range of the system, a new design must feature a vertical aperture to exploit the ambient noise anisotropy. In addition, the system must be inexpensive for use as expendable units for multiple uses such as adjuncts to ocean observatories deployable from air or surface platforms.